This invention relates to a so-called gluing or resin-blending machine, ie a machine used in plants producing wood fibre panels by a dry method (in particular medium density fibreboard or MDF panels), the machine in this case blending wood fibres with a thermosetting liquid glue.
As is well known to the expert of the art, MDF panels are produced by two types of plant, known respectively as a blow-line plant and a resin-blending or traditional plant.
The essential characteristics of a blow-line plant, shown schematically in FIG. 1, will now be described. This shows a grinder 10 which is fed with wood and steam (this feed being indicated by the arrow 12) to reduce the wood to fibres. The fibres produced in this manner leave the grinder 10 through a blow-up valve 14 mixed with steam (relative humidity 100%) and are conveyed via a line 16 to a dryer 18. A liquid glue is injected through the blow-up valve 14 (as indicated in FIG. 1 by the arrow 20) at a pressure of about 6-8 bar, so that a mixture of wood fibres and glue is fed to the dryer 18. Hot gas 26 and air 28 at ambient temperature are also fed to the dryer 18 via a fan 22 and a line 24, to dry the fibres. For this purpose the temperature within the dryer 18 can be varied from 120 to 250xc2x0 C. The fibres dried in this manner are conveyed via a line 30 to a bank of cyclones 32 in which the dry fibre is separated from the steam and gas (formaldehyde) which develop during drying, the steam and gas being discharged to atmosphere, as indicated by the arrow 34.
The fibres leaving the cyclones 32, and having a moisture content varying from 2 to 10%, are fed via a mechanical conveyor 36 to a continuously operating so-called bunker weigher 38 where they are weighed out. The fibres leaving the bunker weigher 38 are conveyed to a pneumatic separator 40 the purpose of which is to remove the xe2x80x9cimpuritiesxe2x80x9d present in the fibre (glue lumps and coarse fibres). At the exit from the separator 40 the fibres are fed by pneumatic conveying 42 to a cyclone 44 feeding a forming machine 48 via a metering belt 46.
This type of plant produces good-quality panels, ie free from glue lumps which would limit their subsequent use (for example making them unsuitable for painting or for xe2x80x9ccladdingxe2x80x9d, ie covering with decorative paper glued to the panel). The mechanical characteristics of these panels fall within the relative regulations. However glue consumption is high (between 120 and 180 kg per m3 of finished panel) because of the fact that the glue is injected at the blow-up valve 14. In this respect, the glue passes, together with the fibres, through the dryer 18 (which operates at high temperature), to hence undergo prepolymerization which reduces its effectiveness.
Even more penalizing for this method is the presence of formaldehyde (contained in the glue) in the gas discharged to atmosphere at 34 after leaving the drying cyclones 32. This means that scrubbers have to be used for this gas together with treatment devices for the resultant water, with consequent considerable plant and operating costs.
A traditional or resin-blending plant is shown schematically in FIG. 2 in which elements similar or identical to those of FIG. 1 are indicated by the same reference numeral plus 100. A grinder is again provided, fed with wood and steam as indicated by the arrow 112. The fibres obtained from the grinder 112, and mixed with the steam (relative humidity 100%), are conveyed through a blow-up valve 114 and along a line 116 to a dryer 118 similar to the dryer 18 of FIG. 1. Hot gas 126 and air 128 at ambient temperature are also fed to the dryer via a fan 122 and a line 124 to dry the fibres. The dried fibres are then conveyed via a line 130 to a bank of cyclones 132 in which the dry fibre is separated from the steam which develops during drying, this being discharged to atmosphere, as indicated by the arrow 134.
The fibres leaving the cyclones 132, and having a moisture content varying from 2 to 10%, are fed via a mechanical conveyor 136 to a bunker weigher 138 where they are weighed out, to be then conveyed to a gluing machine (also known as a resin-blending machine) 120. This gluing machine is essentially a horizontally positioned cylindrical chamber inside which there is a mixing member 223 consisting basically of a coaxial rotating shaft provided with radial paddles. Liquid glue is injected through nozzles into the chamber (as schematically indicated by the arrow 121), it being the task of the mixing member 223 to uniformly distribute the glue throughout the fibre mass. After passing through the entire gluing machine, the fibres are conveyed into a pneumatic separator 140 to separate the xe2x80x9cimpuritiesxe2x80x9d present in the fibre. The fibre is then fed by pneumatic conveying 142 to a cyclone 144 feeding a forming machine 148 via a metering belt 146.
Compared with the preceding, this type of plant has the advantage of low glue consumption and low formaldehyde emission to the atmosphere. However the gluing machine 120 does not distribute the glue with sufficient uniformity throughout the fibre mass, so that this type of plant produces poor-quality panels with the formation of lumps and stains which drastically limit the use of the product obtained. In particular the panels produced cannot be painted or clad. Moreover because of the poor glue distribution, these panels do not present mechanical and engineering characteristics which remain constant with time and are uniform throughout the panel.
Italian patent 1274565, in the name of the present applicant, describes a gluing machine for wood fibre panel production by a dry process, which if used in a traditional plant enables the aforedescribed drawbacks to be overcome. This gluing machine comprises a horizontally positioned hollow cylindrical body, at one end of which there is provided an entry aperture for feeding an air stream which conveys the wood fibre mass within which the liquid glue is to be distributed, at the other end of the cylindrical body there being provided an exit aperture for outflow of the air stream conveying the glue-impregnated fibres. Sprayer means are also provided to spray with glue the fibre mass fed to the gluing machine. Means are also provided to maintain the fibres in proximity to the inner wall of the cylindrical body for a predetermined length as they flow through it. Said sprayer means are positioned coaxially along said length throughout which the fibres are maintained in proximity to the inner surface of the cylindrical body. The means for maintaining the fibres in proximity to said inner wall comprise a pipe having a downstream-facing open end which opens into the cylindrical body in the vicinity of the sprayer means, this pipe extending coaxially in the upstream direction from said end at least for a certain length before leaving the cylindrical body, an air stream being fed into the other end of this pipe.
This gluing machine can comprise a mixing member (for example a motorized shaft provided with paddles) arranged downstream of the position in which the fibres are struck by the jets of glue.
If used in a resin-blending plant in place of traditional gluing machines, the aforedescribed gluing machine enables wood fibre panels to be obtained of substantially better quality than those obtainable with traditional plants provided with a gluing machine. It has however the drawback that the lateral inner wall of the cylindrical body of the gluing machine is very easily fouled because the glue-impregnated fibres tend to adhere to it. This means that frequent plant shut-downs are required for cleaning said inner wall, with consequent serious repercussions on production costs.
The object of the invention is therefore to provide a gluing machine which while enabling optimum quality wood fibre panels (ie suitable for painting or cladding) to be obtained from the relative plant with low glue consumption and minimum formaldehyde emission, does not require frequent cleaning of the inner lateral surface of the gluing machine body.
This object is achieved by the gluing apparatus of the invention, comprising a tubular body and sprayer means to inject a suitable liquid glue into the interior of the tubular body, characterised in that the cross-section through the tubular body encloses an area which increases in progressing from its open upstream end, into which the wood fibres are fed conveyed by an air stream, to its open downstream end, the tubular body comprising a sealed outer wall and an air-permeable inner wall, in the outer wall of the tubular body there being provided a plurality of apertures distributed along this latter to enable additional air streams to be fed into it.
Said air-permeable inner wall is conveniently a wall (for example of steel) provided with a plurality of perforations having a substantially uniform distribution.
From tests carried out it has been found that the gluing apparatus of the invention does not present the drawback of fouling of its inner surfaces. even though it enables fibre panels of optimum quality to be obtained, using only a small quantity of glue, and with minimum release of formaldehyde into the atmosphere by the relative plant.
Said sprayer means are conveniently distributed along the tubular body at least throughout a portion of its length, starting from the downstream end. However coaxial sprayer means could also be used of the type described and illustrated in the aforestated patent 1274565.